Patients 70 years or older presenting to our hospital with an acute femoral neck fracture are eligible for inclusion. 15 patients in each group will be recruited. One group receives a unipolar hemiarthroplasty and 15 receives a bipolar arthroplasty. Tantalum markers are placed around the acetabulum during surgery. Follow-up intervals are at 3, 12 and 24 months with radiostereometric evaluation (radiographs) and clinical evaluation.